Isoenzyme polymorphism in entamoeba histolytica : an epidemiological survey in a rural South African population.
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Date
1989
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Abstract
Isoenzyme characterisation of Entamoeba histolytica into pathogenic and
non-pathogenic zymodemes substantiated previously held views that this
parasite con5titutes two distinct strains or even sub-species that are
morphologically identical but vary in their pathogenicity. A reappraisal of the epidemiology of amoebiasis and investigation of the patho-physiological
relationships between these pathogenic and non-pathogenic zymodemes and their host was therefore indicated.
Only pathogenic zymodemes were isolated from hospitalised patients with
amoebic liver abscess (ALA) and amoebic dysentery (AD). In the amoebiasis
endemic peri-urban population of Durban, I. histolytica occurred at an overall prevalence of 10%. Carriers of non-pathogenic zymodemes
constituted 9% of the population. A key observation was that asymptomatic
infections with pathogenic zymodemes occurred at a prevalence of 1%. Higher prevalence of E. histolytica occurred in association with poor sanitary conditions. Furthermore., both pathogenic and non-pathogenic
zymodemes tended to cluster into family units suggesting person-to-person transmission of the parasite by the faecal-oral route.
Although invasive amoebiasis occurs far more frequently in males than
females (8:1) both pathogenic and non-pathogenic zymodemes are equally
distributed in male and female E. histolytica cyst passers.
Ninety percent of carriers of pathogenic zymodemes spontaneously cleared
their infections and remained asymptomatic throughout the study period
of 2 years while 10% developed AD which required treatment with
metronidazole. No spontaneous changes in zymodemes from the non-pathogenicto the pathogenic type was observed in a longitudinal study.
The serological response of asymptomatic carriers of pathogenic zymodemes (100% seropositive) was identical to that of patients with ALA or AD with a high proportion (94-100%) of them being strongly seropositive.
The prevalence of seropositivity amongst subjects who were not infected
by E. histolytica (13% seropositive) was not statistically different (p>0,5) from that of the random population of this endemic area (19%
seropositive) and carriers of non-pathogenic zymodemes (21% positive);
the prevalence of strongly seropositive reactions among this group was
only between 2-4%. It is concluded that a positive serological response
is directly due to past or present contact with pathogenic zymodemes.
This is further substantiated by the observation that the proportion of
seropositive subjects was found to increase dramatically in a population
near Cape Town where an outbreak of invasive amoebiasis (ALA and AD)
occurred indicating a high prevalence of pathogenic zymodemes in this
community. Another community in northern Transvaal (Gazankulu) where ALA and AD does not occur was, as expected, uniformly seronegative.
Axenic growth of pathogenic zymodemes was possible but could not be
accomplished with the non-pathogenic zymodemes. Even though monaxenic
growth together with Trypanosoma cruzi was possible with both strains,
the pathogenic zymodemes tended to grow more prolificly. No zymodeme
changes from non-pathogenic to pathogenic and vice versa were observed
with such changes in culture conditions.
Cyst production by the pathogenic zymodemes in vivo was confirmed
experimentally, thereby demonstrating the ability of pathogenic E.
histolytica to independently complete their life-cycle thus giving it
the ability to propagate itself successfully as a species.
Description
Thesis (M.D.)-University of Natal, Durban, 1989.
Keywords
Entamoeba histolytica--South Africa., Theses--Infectious diseases.